Formations containing oil or gas in many cases also have water present. These formations will produce water along with the hydrocarbons; however, due to the heterogeneity of many formations, water will flow preferentially from certain intervals. When low viscosity treating fluids are injected into the matrix of the formation, below the fracturing pressure, such as in matrix acidizing to try to increase the production of fluids from the reservoir, there is a tendency for the treating fluid to enter preferentially into the intervals with the highest water saturation, due to the lower viscosity and higher mobility of water compared to oil in most reservoirs. The increase in water production after these treatments is usually greater than any associated increase in oil or gas.
A relative permeability modifier (RPM) is generally a compound, usually a polymer, which when injected into a formation will adsorb onto the surface of the rock matrix and significantly reduce the relative permeability of the formation to water. Following RPM treatment, the formation should produce oil and/or gas at about the same or at an increased rate as prior to the treatment, but with less water due to the reduced effective permeability of the formation to water. Reducing the relative permeability to water in intervals which have high water saturation should, in theory, decrease water production from these intervals after the treatment.
It has also been known to use a diverter stage to divert a treating fluid stage away from intervals in which the fluid is initially being injected, which are usually the intervals with the highest matrix permeability and/or highest water saturation. It has proven difficult to divert a treating fluid away from the high-permeability matrices. As a consequence the failure rate in diversion treatments is high, and it has been common, due to excessive water production, to abandon intervals unsuccessfully treated with diversion.
Cationic polyacrylamides have been proposed for use in RPM treatments, but with only limited success. It has been reported, for example, that cationic polyacrylamide in freshwater aqueous solution binds poorly to formation surfaces. The prior art has thus attempted to adsorb the cationic polyacrylamide onto the formation rock in a saline solution at a low pH (acidic) for best RPM performance; however, even where high salinity and low pH are appropriate applications, many RPM water-control compounds are unstable at these conditions and/or they may degrade with increased temperature, further limiting their application. The art continues to search for RPM water-control treatment fluids and methods suitable for a wider range of application conditions.